The P-T Conditions of Late Stage Diagenesis and Low Grade Metamorphism in the Taconic Belt of the Gaspe Peninsula From Fluid Inclusions: Preliminary Results

Islam, Shofiqul; Hesse, Reinhard

doi:10.4095/109318

Cited by 4 publications

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“…Regional patterns of the degree of diagenesis and low-grade metamorphism of pelitic rocks in the Gasp6 Peninsula were defined by Islam et al (1982), Duba and Williams-Jones (1983), Islam and Hesse (1983), and Hesse and Dalton (1991). Islam et al (1982) observed a simple regional pattern with an increase of diagenetic and metamorphic grade toward the McGerrigle pluton due to thermal alteration, superimposed on a northward trend of increasing grade that parallels a decrease in rock ages in the area east of the pluton.…”

Section: Materials and Experimental Techniquesmentioning

confidence: 99%

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Prograde Transitions of Corrensite and Chlorite in Low-Grade Pelitic Rocks from the Gaspé Peninsula, Quebec

Jiang

1994

Clays and clay miner.

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Abstract-A prograde sequence ofcorrensite and chlorite in pelitic rocks of the diagenetic zone, anchizone, and epizone (illite crystallinity indices = 0.17-0.58~ of the Gasp6 Peninsula, Quebec, was studied by analytical and transmission electron microscopy (AEM and TEM). The data collectively suggest that diagenesis/metamorphism of chlorite and corrensite follows a sequence of phase transitions, compositional homogenization, and recrystallization, approaching a state of equilibrium for which chlorite is the stable phase.Corrensite occurs as coalescing, wavy packets of layers intergrown with chlorite and illite in the diagenetic and low-grade anchizonal rocks. Intergrowths of discrete chlorite and corrensite crystals, interstratified packets of chlorite and corrensite layers, terminations of smectite-like layers by chlorite layers, and 2-3 repeats of R2-and R3-ordered chlorite-smectite mixed layers occur. These materials are alteration products of detrital biotite or other precursor phases like trioctahedral smectite. The crystal size and proportion of corrensite decrease significantly from the diagenetic zone to the anchizone. Deformed corrensite is crosscut by straight packets of chlorite and corrensite in the diagenetic sample. Some chlorite occurs as discrete, euhedral to subhedral crystals intergrown with or enclosed by other phases in the absence of corrensite. The crystal size of chlorite and definition of crystal boundaries increase whereas density of crystal imperfections and randomness in orientation decrease with increase in grade of diagenesis/metamorphism. Crystals that are kinked or bent, or display gliding along (001) form low-angle boundaries with relatively defect-free crystals, implying deformation during crystal growth. Abundant well-defined low-angle boundaries associated with dislocations are observed in the higher grade rocks, consistent with a stage of readjustment of crystal boundaries during crystal growth. The AEM analyses show that the corrensite has lower Fe/(Mg + Fe) and A1/(Si + A1) than the coexisting chlorite in the diagenetic sample, and that the ranges of composition of chlorite of different grades overlap and become smaller with increasing grade, implying prograde homogenization.The data imply that corrensite is a unique phase that is metastable relative to chlorite: its conversion to chlorite occurred at a grade as low as that of the high-grade diagenetic zone. The textural relations suggest that the metamorphic crystallization and recrystallization were coeval with deformation processes due to tectonism, partially modified by subsequent contact metamorphism. The data, combined with those of previous reports, suggest that the Gasp60rdovician rocks constitute a part of a regional distribution of trioctahedral phyllosilicate-rich rocks in the northern Appalachians. The regional occurrence of abundant chloritic minerals is thus directly related to a specific tectonic re#me with precursor sediments largely derived from an andesitic arc system(s).

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Section: Materials and Experimental Techniquesmentioning

confidence: 99%

“…The pattern shown in Figure 1 uses Kisch's limits instead of those defined by Hesse and Dalton (1991). The temperatures of diagenesis and metamorphism of the region were estimated to be ~80~ in the diagenetic zone and ~260"C in the epizone based on fluid inclusion data (Islam and Hesse 1983;Hesse and Dalton 1991).…”

Section: Materials and Experimental Techniquesmentioning

confidence: 99%

Prograde Transitions of Corrensite and Chlorite in Low-Grade Pelitic Rocks from the Gaspé Peninsula, Quebec

Jiang

1994

Clays and clay miner.

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“…A revision of previously published fluid inclusion data for the Taconian belt (Islam & Hesse, 1983) by C. Yang and J. Mullis (pers. comm.)…”

Section: F L U I D I N C L U S I O N D a T Amentioning

confidence: 99%

“…Using these techniques, Islam ef al. (1982) and Islam & Hesse (1983) mapped the northern part of Gasp6 Peninsula in terms of grades and were able to delineate regions of pre-and syn-or post-orogenic alteration. Some of their earlier interpretations will be discussed and in part reassessed in this paper.…”

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confidence: 99%

Diagenetic and low‐grade metamorphic terranes of Gaspé Peninsula related to the geological structure of the Taconian and Acadian orogenic belts, Quebec Appalachians

Hesse

Dalton

1991

Journal Metamorphic Geology

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Illite crystallinity (IC) measurements, determination of the proportion of 2M mica-polytypes and organic-matter reflectance measurements establish regional diagenetic/low-grade metamorphic trends for the Taconian and Acadian belts of GaspC Peninsula.IC varies as a function of many factors besides maximum burial temperature and heating time. Correlation between IC and %2M illite polytypes for the Fortin Group and Temiscouata Formation suggests (i) that the amount of high-grade detrital mica in the samples is low, and (ii) that IC can be used with some confidence as an estimator of regional thermal maturation levels. Correlation of these parameters with available organic reflectance values further supports this assumption. The illites of the Temiscouata and western Fortin groups are mostly phengitic in composition, whereas in the eastern outcrop belt they are more Mg-and Fe-rich (celadonitic), but generally also of lower grade and lower 2M content. The d(060) values for illites measured on the unorientated <2-pm fraction of samples fall between 1.502 and 1.503 A (range: 1.500-1.504 A), indicating relatively low octahedral occupancy by Mg and Fe (between one-fifth and one-third of the available spaces). Pyrophyllite and paragonite were not detected. Chlorites are Fe-rich and ripidolitic.The IC map for the Acadian belt of the peninsula displays general congruence between IC contours (2200 sample points) and structural trends for the 27,000-km' area. The highest grades (anchimetamorphic) are associated with the oldest rocks (Honorat and Matapedia groups) exposed in the cores of major anticlines. Anchimetamorphic grades associated with the western outcrop belt of the Lower Devonian Fortin Group require 7-8 km of subsidence to accommodate sufficient thickness of overlying younger rocks (on top of 4-5 km of Fortin Group deep-water clastics) to explain the grades in terms of burial metamorphism assuming a geothermal gradient of 30" C km-'. The lowest-grade diagenetic rocks occupy a large area in the northeastern part of the peninsula, smaller areas in the northwestern part of the Acadian belt, in the centre of Chaleurs Bay synclinorium, and in the Ordovician Mictaw Group. The contact between the Taconian and Acadian belt is marked by a distinct maturation discontinuity. The Grand Pabos fault juxtaposes rocks of contrasting maturation levels (Matapedia Group against Fortin Group) in the west, but shows no maturation offset further east in the Honorat Group. The fault zone limiting the Fortin Group in the north is also associated with a major IC jump.

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TEM and XRD determination of crystallite size and lattice strain as a function of illite crystallinity in pelitic rocks

Jiang¹,

Peacor

Árkai

et al. 1997

Journal Metamorphic Geology

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A B S TR A CT Average crystallite size and mean-square strain of illite in rock specimens and clay separates were measured independently in TEM images and by single-line Fourier (Voigt method) profile analysis of the c. 1 nm peak of XRD patterns for a prograde sequence of pelitic rocks (illite crystallinity indices=0.17-0.58°D 2h) from the Gaspé Peninsula, Quebec. The TEM-determined crystallite sizes in clay separates approximate those determined by Fourier profile analyses and those calculated from illite crystallinity indices by the Scherrer equation, with the exception of the diagenetic sample. The crystallite sizes and mean-square strains of illite in rock samples exhibit a trend similar to that determined by profile analyses, but the average crystallite sizes are up to five times larger than those measured for clay separates.TEM images show that all rock samples have a wide range of crystallite sizes, and the proportions of larger crystallites increase with metamorphic grade. The diagenetic illite is defect-rich, fine-grained (mean thickness by volume=c. 70 nm), 1M d material. Anchizonal illite tends to occur as separate aggregates of small 1M d and larger 2M 1 crystals (c. 200 nm), comprising arrays of subparallel coalescing packets. The epizone sample has thick (c. 400 nm), defect-free crystals of muscovite occurring in stacks of parallel layers, or subhedral crystals intergrown with large-angle boundaries. Cleaved crystals that are free of intracrystalline layer terminations are dominant in clay separates of all samples, having ranges of smaller sizes with volume-average thicknesses of c. 43, 43, and 81 nm (c. 14, 28, 67 nm by the Voigt method), respectively, for the three zones.The results suggest that illite crystallinity indices do not provide a direct measure of a single microstructural state of illite in rocks, although they yield consistent limits for average crystallite sizes for the anchizone (23 & 48 nm here). Therefore, they serve as a general parameter of the degree of recrystallization on a relative basis, in part because the contributions of all peak-broadening variables (mixed layering, size and strain) decrease regularly with prograde regional metamorphism of pelites. The microstructural changes caused by rock disaggregation are probably a function of those variables as well. The data collectively demonstrate a trend from metastable, defect-rich, small crystals towards a stable assemblage of larger, defect-free crystals, through dissolution of strained crystals and neocrystallization, consistent with the Ostwald step rule.

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The P-T Conditions of Late Stage Diagenesis and Low Grade Metamorphism in the Taconic Belt of the Gaspe Peninsula From Fluid Inclusions: Preliminary Results

Cited by 4 publications

References 0 publications

Prograde Transitions of Corrensite and Chlorite in Low-Grade Pelitic Rocks from the Gaspé Peninsula, Quebec

Prograde Transitions of Corrensite and Chlorite in Low-Grade Pelitic Rocks from the Gaspé Peninsula, Quebec

Diagenetic and low‐grade metamorphic terranes of Gaspé Peninsula related to the geological structure of the Taconian and Acadian orogenic belts, Quebec Appalachians

TEM and XRD determination of crystallite size and lattice strain as a function of illite crystallinity in pelitic rocks

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